Network Working Group D. Singer
Request for Comments: 5450 Apple Computer Inc.
Category: Standards Track H. Desineni
Qualcomm
March 2009
Transmission Time Offsets in RTP Streams
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Abstract
This document describes a method to inform Real-time Transport
Protocol (RTP) clients when RTP packets are transmitted at a time
other than their 'nominal' transmission time. It also provides a
mechanism to provide improved inter-arrival jitter reports from the
clients, that take into account the reported transmission times.
Singer & Desineni Standards Track [Page 1]RFC 5450 RTP Transmission Offsets March 2009Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
3. Transmission Offset . . . . . . . . . . . . . . . . . . . . . . 3
4. Extended Jitter Reports . . . . . . . . . . . . . . . . . . . . 5
5. Signaling (Setup) Information . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In the Real-time Transport Protocol (RTP) specification [RFC3550],
network jitter calculations are based on the presumption that packets
are transmitted essentially in accordance with their RTP timestamps.
This must be true, of course, on average over longer time intervals,
as the client is playing the packets out according to those
timestamps. However, for individual packets, this may not be true
under some circumstances, such as:
o When the data rate of the stream is bursty, such as with video
where I-frames may be significantly larger than P or B frames,
traffic smoothing may need to be applied to maintain an
appropriate data rate.
o In video that has forward-decode dependencies, frames may need to
be transmitted in decoding order (the sequence number order) but
with, of course, presentation timestamps. Under these
circumstances, the transmission time of a frame sent early in
sequence does not correspond to its RTP timestamp.
o When retransmissions are sent, the retransmitted packet clearly
has a different actual transmission time from the original, even
though they share the same timestamp.
Under some circumstances, it can help the receiver, or intermediate
network elements, to know the actual transmission time of the packet.
This RTP header extension element allows the communication of this
information.
The RTP specification does not define a transmission timestamp; nor
does this specification. This specification merely provides
information on the relationship between the relative transmission